In general, organic electroluminescence (EL) devices comprise an anode, an organic light emitting layer and a cathode. Within the organic light emitting layer, excitons are formed in a process called exciton formation or polaron recombination whereby oppositely charged carriers, i.e., electrons and holes, recombine. The radiative decay of the excited excitons produced by the recombination process to their ground state results in electroluminescence.
The organic light emitting layer may be formed using low molecular weight electroluminescent materials. Consequently, this will enable luminescence over a wide wavelength range from the UV region to the IR region. For example, Welter et al., 421(6918) NATURE, 54-57 (2003) describes an EL device comprising a film of a conjugated polyphenylenevinylene (PPV) polymer as the light-emitting layer. Additionally, PPV-type polymers have been developed which are soluble in an organic solvent, have high chemical stability and a high luminous efficiency.
Although poly(fluorene) compounds having better performance than the PPV-type polymer as the light-emitting layer material have been developed, color purity, however, is still unsatisfactory. See U.S. Pat. No. 5,900,327. Therefore, electroluminescence materials capable of improving the color purity of EL devices are being developed. For example, U.S. Pat. Nos. 5,763,636, 5,621,131 and 5,859,211, disclose spirobifluorene or a hetero atom containing spirofluorene as electroluminescence materials capable of improving the property profile of EL devices.
Organic electroluminescence devices manufactured using the existing light-emitting materials, however, are still required to be further improved, particularly for efficiency, color purity and the like.